Chromium isotope heterogeneity in the mantle
Chromium isotope heterogeneity in the mantle
复制标题
地幔中铬同位素异质性
DOI：
10.1016/j.epsl.2017.01.045
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发表时间：
2017
影响因子：
5.3
通讯作者：
Mock Timothy D.
中科院分区：
文献类型：
--
作者：
Xia Jiuxing;Qin Liping;Shen Ji;Carlson Richard W.;Ionov Dmitri A.;Mock Timothy D.
为了更好地约束硅酸盐地球的铬同位素组成和研究高温地质过程中潜在的铬同位素分馏作用，我们分析了不同地质背景下不同类型的地幔包体的铬同位素组成，包括来自蒙古中部的可育和难熔的克拉通外尖晶石和石榴石橄榄岩、辉石岩脉、交代尖晶石二辉橄榄岩及其伴生玄武岩、来自北中国的尖晶石二辉橄榄岩和方辉橄榄岩，以及来自西伯利亚和南非的克拉通尖晶石和石榴石橄榄岩。橄榄岩的δ5 3CrNIST979值范围为−0.5 1±0.0 4‰(2SD)~+0.75±0.0 5‰(2 SD)。结果表明，大多数地幔橄榄岩的δ-53Cr与Al2 O3和CaO含量呈轻微负相关，这可能反映了地幔橄榄岩部分熔融过程中的铬同位素分馏。然而，在蒙古橄榄岩中测得的高度可变的铬同位素组成并不是仅由部分熔融造成的。相反，这些样品的铬同位素组成范围很大，很可能反映了熔体渗流过程中的动力学分馏。熔体渗流过程中的化学扩散导致轻铬同位素优先进入熔体。来自蒙古的两个尖晶石辉橄榄岩脉具有极轻的δ53Cr值，分别为−1.36±0.04‰和−0.77±0.06‰，这是迄今报道的地幔岩石中最负的铬同位素组成。这两个网辉岩脉可能代表了同位素轻熔体的结晶产物，也可能使该地区的一些橄榄岩交代。南部非洲蚀变程度较大的石榴石橄榄岩的δ5 3Cr值变化范围为−0.35±0.0 4‰(2SD)~+0.12±0.0 4‰(2SD)，并随LOI(失火率)的增加而增加，反映了二次蚀变使δ5 3Cr值向更正值移动。经部分熔融和交代作用校正后，原始、富饶的上地幔的铬同位素组成为δCr53=−0.14±0.12‰。这一值与之前对疯牛病的估计值(−0.12±0.10‰)一致。
To better constrain the Cr isotopic composition of the silicate Earth and to investigate potential Cr isotopic fractionation during high temperature geological processes, we analyzed the Cr isotopic composition of different types of mantle xenoliths from diverse geologic settings: fertile to refractory off-craton spinel and garnet peridotites, pyroxenite veins, metasomatised spinel lherzolites and associated basalts from central Mongolia, spinel lherzolites and harzburgites from North China, as well as cratonic spinel and garnet peridotites from Siberia and southern Africa. The δ 53 Cr NIST 979 values of the peridotites range from− 0.51±0.04‰(2SD) to+ 0.75±0.05‰(2SD). The results show a slight negative correlation between δ 53 Cr and Al 2 O 3 and CaO contents for most mantle peridotites, which may imply Cr isotopic fractionation during partial melting of mantle peridotites. However, highly variable Cr isotopic compositions measured in Mongolian peridotites cannot be caused by partial melting alone. Instead, the wide range in Cr isotopic composition of these samples most likely reflects kinetic fractionation during melt percolation. Chemical diffusion during melt percolation resulted in light Cr isotopes preferably entering into the melt. Two spinel websterite veins from Mongolia have extremely light δ 53 Cr values of− 1.36±0.04‰ and− 0.77±0.06‰, respectively, which are the most negative Cr isotopic compositions yet reported for mantle-derived rocks. These two websterite veins may represent crystallization products from the isotopically light melt that may also metasomatize some peridotites in the area. The δ 53 Cr values of highly altered garnet peridotites from southern Africa vary from− 0.35±0.04‰(2SD) to+ 0.12±0.04‰(2SD) and increase with increasing LOI (Loss on Ignition), reflecting a shift of δ 53 Cr to more positive values by secondary alteration. The Cr isotopic composition of the pristine, fertile upper mantle is estimated as δ Cr 53=− 0.14±0.12‰, after corrections for the effects of partial melting and metasomatism. This value is in line with that estimated for the BSE (− 0.12±0.10‰) previously.